Conventional light-emitting devices (such as an LED device) in which semiconductor crystals such as III family nitride compound semiconductors are grown on a light-transmitting growth substrate such as a sapphire have been known. When such an LED device is to be mounted on a circuit pattern, the light-transmitting growth substrate can be used to produce a flip-chip type LED device in which light can be extracted from the growth substrate side.
Japanese Patent Application Laid-Open No. 2003-243709 describes an LED device. Such an LED device is shown in FIGS. 1 and 2. A first contact layer is deposited on a sapphire substrate, and 9 (nine) circular n electrodes are formed to be arranged in a rectangular matrix manner. Further, a rectangular light-emitting portion is formed in a manner such that a light-emitting semiconductor layer surrounds the respective circular n electrodes. In the LED device, the light-emitting semiconductor layer including a light-emitting layer interposed between cladding layers, a second contact layer and a p electrode are stacked on the first contact layer in this order. This LED device can be flip-chip mounted via bumps (not shown) to be electrically connected at the side where the p electrode and the n electrodes are exposed, and when a current is supplied so that the device emits light, heat generated from inside of the light-emitting semiconductor layer during the light emission can be effectively dissipated through the bumps. In this LED device, the light from the light-emitting layer corresponding to the p electrode can be emitted through the sapphire substrate so as to surround the n electrodes. (See the arrows in FIG. 2.)
Examples of such a flip-chip type LED device may include: one having comb-shaped cathode and anode electrodes disposed in an alternate manner to cause a rectangular light-emitting surface to emit light entirely (see, for example, Japanese Patent Application Laid-Open No. 2007-258276); and one having a comb-shaped p electrode with wide tooth portions and thin n electrodes each disposed in between the adjacent wide tooth portions of the p electrode to cause a rectangular light-emitting surface to emit light entirely (see, for example, Japanese Patent Application Laid-Open No. 2007-300134), and the like.
LED devices have been developed to be utilized as a light source in the form of an LED module for a vehicle lighting unit. In particular, a common vehicle LED module may have a directivity in a Lambertian distribution, and accordingly, an LED module having a rectangular light-emitting surface formed by arranging a plurality of rectangular LED devices in line in the same direction can be typically utilized in a vehicle headlamp. When a vehicle headlamp utilizes a headlamp optical system using a convex lens and a reflector in combination, such an LED module with a rectangular light-emitting surface can be used to be disposed at or near the rear-side focal point of the optical system.
A vehicle headlamp is required to have high luminance, and an LED module for use in a headlamp is required to include an LED device capable of receiving a large amount of current and have a high light utilization efficiency. In view of this, a flip-chip type LED device can provide benefits when utilized as an LED module for a vehicle headlamp since a large amount of current can be supplied via a plurality of bumps, no wiring, which otherwise hinders the light emission, is required, and the distance between the adjacent devices can be shortened.
Further, in the technical field of a vehicle headlamp to be disposed on both right and left sides of a vehicle front body, a passing light distribution pattern or a low beam light distribution pattern should be formed so that any glare light to a driver in an opposite vehicle cannot be generated by the headlamp during the vehicle passing each other. This light distribution pattern can be formed of a horizontally wide luminance distribution in order to ensure the front visibility for a driver who is driving the vehicle. Such a low beam light distribution pattern (hereinafter, simply referred to as a “light distribution pattern”) should have a cut-off line including an opposite-lane-side cut-off line and an own-lane-side cut-off line that extends at an upper level or in an obliquely upward direction from an elbow point (changing point) with respect to the opposite-lane-side cut-off line. Accordingly, the vehicle headlamp can be configured to provide an appropriate luminance distribution near and below the cut-off line, so that the wider area of the road in front of the vehicle body can be illuminated with the light from the vehicle headlamp.
When such a vehicle headlamp with the above-mentioned light distribution pattern includes an LED module, the LED module is required to provide more uniform luminance distribution as well as higher luminance. In order to satisfy the requirements, such an LED module may be designed to include a plurality of the above flip-chip type LED devices at a higher density. However, in this case, light-emission unevenness between the respective LED devices, leading to lower the production yield. It would be possible to expand the surface area of each LED device. However, since the flip-chip type LED device can emit light from the light-emitting layer corresponding to the p electrode through the sapphire substrate while the light is emitted so as to surround the n electrodes (cathode electrodes), the unevenness of the luminance distribution of each LED device may be problematic. Namely, there is the problem of the luminance distribution unevenness due to the difference in luminance between the areas where the n electrodes are arranged without the light-emitting semiconductor layer and the remaining area.
Further, the illuminance uneveness in a light distribution pattern for a headlamp is affected less by horizontal stripes than by longitudinal stripes generated by its optical system. This is because the illuminance uneveness in longitudinal stripes generated in an LED module can be alleviated more by the optical system than that in horizontal stripes, resulting in smaller illuminance uneveness in the light distribution pattern. Therefore, it is beneficial to suppress the generation of horizontal stripes in the luminance distribution in an LED module.